1. Field of the Invention
The present invention relates to a DC offset calibration system that corrects a DC offset which emerges when an interfering signal is input in a direct conversion type radio receiver.
2. Description of the Related Art
Existing mobile phones include a dual band mobile phone compatible with signals of two different frequency bands, for example bandwidths of 880 to 915 MHz allocated to GSM (Global System for Mobile Communication) and 1710 to 1785 MHz allocated to DCS (Digital Cellular System). Some of such dual band mobile phones include a frequency synthesizer so as to handle two frequency bands by switching the frequency of a local signal.
Lately, however, there is a growing demand for a triple band mobile phone that is compatible with signals of 1850 to 1915 MHz allocated to PCS (Personal Communication System), in addition to the GSM and DCS. Thus, compatibility with still more frequency bands will be required from the mobile phone in the near future. For demodulating a signal received by a radio equipment compatible with a plurality of frequency bands, it is advantageous to employ a direct conversion type high-frequency semiconductor IC, from the viewpoint of micronization in dimensions of the radio equipment and manufacturing cost. The direct conversion system eliminates the need to provide an intermediate frequency filter, since a RF input signal is directly converted into a low-frequency base band signal, unlike a conventional system which requires the intermediate frequency.
The frequency conversion is performed by mixing a RF input signal and a local signal of the same frequency as the RF input signal frequency, in a mixer circuit. Under the direct conversion system, however, if a secondary nonlinear distortion is present in the mixer circuit, a DC offset emerges in an output base band signal when the level of the input signal is high. Such phenomenon will be described in details referring to FIGS. 4 and 5.
FIG. 4 shows a spectrum of a RF input signal. In FIG. 4, reference numeral 501 designates a low level desired signal having a center frequency equal to a local signal frequency fLo, and 502 designates a high-level interfering signal having a higher frequency fINT. When the RF input signal accompanied with such high-level interfering signal is input to a mixer circuit, the spectrum of the output signal as shown in FIG. 5 appears in the output of the mixer. In FIG. 5, numerals 601, 602 respectively designate the components of the desired signal and the interfering signal generated as the mixer output upon frequency-conversion of the RF input desired signal 501 and the interfering signal 502. Numeral 603 designates a DC offset generated by the high-level interfering signal when a secondary nonlinear distortion is present in the mixer circuit. Accordingly, the direct conversion system has a drawback that the receiving sensitivity is degraded because of the presence of the DC offset 603 in the frequency band of the desired signal 601 in the mixer output. Provided that the mixer circuit is constituted of a differential circuit in a perfectly symmetrical differential balance, a secondary nonlinear distortion is not generated. However, the perfect symmetry of the elements constituting the differential circuit cannot be practically achieved because of manufacturing variation among individual units, and hence it is impossible to eliminate the secondary nonlinear distortion.
To solve this problem, a technique of correcting the DC offset generated by the secondary nonlinear distortion has been proposed.
Referring now to FIG. 6, description will be made on the technique of detecting an interfering signal contained in a RF input signal, to thereby correct a DC offset generated in a mixer output, according to patented document 1. In FIG. 6, numeral 403 designates a mixer circuit, and 401, 402 a switching cell and a RF input cell constituting the mixer circuit 403, respectively. The RF input signal input from RF input terminals 413, 414 is amplified in the RF input cell 402, and the amplified RF signal is mixed with a local signal input from local input terminals 411, 412, in the switching cell 401, thus to be converted to an IF signal. The IF signal thus converted is output from output terminals 409, 410. The direct conversion system is also called a “zero IF system”, since the center frequency of the IF signal is a direct current.
The switching cell 401 includes bipolar transistors Q1, Q2, Q3 and Q4. If all the transistors Q1, Q2, Q3 and Q4 were of an identical characteristic, the differential circuit would have a perfectly symmetrical balance. Practically, however, the transistors Q1, Q2, Q3 and Q4 have a different characteristic from one another shifted from an ideal characteristic, because of the manufacturing variation. This leads to generation of the secondary nonlinear distortion during the conversion of the RF input signal to the IF signal, which results in generation of the DC offset in the mixer output as shown in FIG. 6. As is commonly known, the DC offset is proportionate to the square of the input signal intensity, and hence the DC offset in the output becomes greater when the level of the interfering signal contained in the input signal is higher.
In FIG. 6, numeral 408 designates a DC offset corrector. The DC offset corrector 408 includes a detector 406 that detects the RF input signal and outputs a detecting signal, a controller 405 that outputs a control signal according to the detecting signal, and a correction generator 404 that generates a correction signal according to the control signal from the controller so as to reduce the DC offset in the mixer output terminals 409, 410. The DC offset corrector 408 serves to cause variation of the correction signal according to the intensity of the RF signal input to the mixer circuit 403, thus to cancel the DC offset in the mixer output.
Here, the secondary nonlinear distortion in the mixer circuit 403 is different, owing to the manufacturing variation. Since the mixer circuit 403 has a different characteristic among individual units, DC offset corrector 408 also serves as a user interface 407 for adjusting the control signal generated by the controller 405.
[Patented document 1] U.S. Pat. No. 6,535,725
In such a DC offset calibration system, an additional frequency synthesizer for generating an interfering signal is indispensable, apart from a frequency synthesizer that generates a local signal. The two frequency synthesizers have to oscillate in very close frequencies to each other, in order to satisfy the interfering signal standard of a desired radio system. In this case, interference emerges in the respective voltage control oscillators, which causes degradation in C/N (carrier/noise) characteristic and emergence of a spurious, thus impeding adequately precise DC offset calibration.
Also, incorporating such a DC offset calibration system in a radio system is against the demand for further micronization and cost reduction of the radio system.
Further, a radio system that operates in a plurality of bands includes the same number of mixers corresponding to the respective bands. Accordingly, each of the mixers have to be individually calibrated for the DC offset, for the same reason. Consequently, an interfering signal of a different frequency has to be input to each of the mixers.